This invention relates generally to digital receivers and more particularly to an improved method and system for down-converting a signal.
Communication systems send signals to communicate information, such as voice, image or other data, from one communication device to another. In many applications, the signal is modulated into an analog transmission signal that acts as a carrier wave to communicate the information. The analog transmission signal may be communicated in several different forms. For example, the analog transmission signal may be an electrical signal as used in a copper wire telephone transmission line. Alternatively, the analog transmission signal may be a radio frequency as used in wireless communication systems.
A typical wireless communication system generally comprises a communication device, such as a base station, satellite or the like, that communicates with another communication device over a specific radio frequency band. Conventional communication devices receive the analog transmission signal with a receiver, such as a superheterodyne receiver. A typical superheterodyne receiver includes a digital or analog down-converter that mixes the intermediate frequency signals down to baseband, or a lower intermediate frequency, and low-pass filters the mixed signals to obtain the desired result.
Digital down-converters generally outperform analog down-converters and are less sensitive to variations in parameters such as time, temperature and frequency. In addition, as digital technology advances, digital down-converters are requiring less power and can be manufactured at a lower cost. However, digital down-converters require high speed digital multipliers in order to function at a high sample rate. Thus, it has been difficult to take advantage of the performance of digital down-converters over a wide frequency range because of the correspondingly high sample rate which requires high speed digital multipliers to perform the mixing function.
For applications in which the signal bandwidth is significantly less than the tuning bandwidth, mixing and filtering are generally followed by sample rate decimation so that the sample rate is appropriate for the signal bandwidth. For applications in which the signal bandwidth is wide in comparison to the tuning bandwidth, previous down-converters have used a parallel architecture that replaces one relatively high speed multiplier with a plurality of slower multipliers. Finally, for channelizer applications that require multiple simultaneous signal channels, previous down-converters have used a polyphase filter bank with modulated filter coefficients to channelize the signal. Thus, previous down-converters require relatively low sample rates, high speed multipliers, numerous multipliers in parallel, and/or complex filtering.
In accordance with the present invention, a method and system for down-converting a signal are provided that substantially eliminate or reduce disadvantages and problems associated with previously developed systems and methods. In particular, the present invention decimates the sample rate of the signal before filtering and mixing to digitally tune a signal without high speed multipliers or complex filtering.
In one embodiment of the present invention, a system is provided for down-converting a narrow band signal that includes a polyphase filter having a specified number of filter branches. The polyphase filter is operable to decimate the signal by a decimation factor based on the specified number of filter branches. A channel mixer is coupled to the filter. The channel mixer is periodic with a period of twice the specified number of filter branches.
In another embodiment of the present invention, a method for down-converting a wide band signal is provided that includes mixing the signal with an analog mixer to produce a mixed signal. The mixed signal is band-pass filtered to produce a band-pass filtered signal. The band-pass filtered signal is converted to a digital signal. The digital signal is decimated with a polyphase filter. The polyphase filter has a specified number of filter branches. A decimation factor for decimating the signal is based on the specified number of filter branches. The digital signal is filtered with the polyphase filter to produce a plurality of filter outputs. The filter outputs are mixed with a channel mixer. The channel mixer is periodic with a period of twice the specified number of filter branches.
Technical advantages of the present invention include providing an improved method and system for down-converting a signal. In particular, decimation of the data is performed before filtering, and filtering is performed before mixing. As a result, complex filtering is avoided and slower multipliers and filters may be used. This reduces the overall cost of the down-converter. Additionally, given a maximum technology limit for mixer speed, total tuning bandwidth is significantly increased.
Other technical advantages will be readily apparent to one skilled in the art from the following figures, description, and claims.